Thianthrene radical cations formation

Intermediates in the thermal oxidations of thioethers with NO are generally difficult to detect owing to the limiting reaction rates. The exceptions are oxidations of strong sulphur-containing donors derived from thianthrene. The stoichiometric oxidation of thianthrene in dichloromethane at -78 °C produces a pink-coloured solution ( max =-544 nm) that finally leads to a pale-yellow solution from which thianthrene 5-oxide was isolated in 94% yield. This process involves the intermediate stage of the thianthrene radical cation formation [23] ... 

Formation of 7r-cation radicals has also been suggested [35] in the reaction of stable enols with thianthrene radical cation and other one-electron oxidants. Thus enol 10 on treatment with thianthrene radical cation affords benzofuran derivative 11 in 87 % yield. The initial step in this reaction is suggested to be one-electron transfer forming the cation radical of 10, which has been unequivocally identified in a related system [36]. 

A more detailed study" has shown that this oxidation occurs as a stepwise process with the second oxidation potential 20 mV lower than the first. Such anomalous electrochemical behavior of 10 presents more evidence for the formation of a S-S dication." The first evidence that 10 can be oxidized chemically to a stable disulfonium dication was obtained by Shine and Kette." While investigating the oxidative properties of the thianthrene radical cation... 

Molecular ions obtained from thianthrenes are normally the base peak in their mass spectra. The principal fragmentation involves loss of sulfur (87PS377), and this is interpreted as formation of a dibenzothiophen radical cation (16). Further loss of sulfur then occurs. CSH is lost from both the dibenzothiophen fragment ion and from the molecular ion species such as 17, from the parent ion, are proposed (74JHC287). The mass spectroscopic fragmentation pattern of fluorothianthrenes is comparable (720MS373). 

Azoadamantane exposed to 2 mol equivalents of T CIO4 at room temperature rapidly and quantitatively evolved nitrogen, and thianthrene and products derived from the adamantyl cation were obtained. Equations (38)-(40) (AA, azo-adamantane Ad, adamantane) make clear why 2 mol equivalents of the radical oxidant are required (85JA2561). The comparable interaction of T with phenylazotriphenylmethane and di-ter/-butyl diazene, using a 2 1 ratio of radical cation to substrate, also leads to the formation of thianthrene and nitrogen (85PS111). 

Further interesting examples of C-S bond formation involve the reaction of previously prepared (or in situ anodically generated) thianthrene or phenothiazine radical cations with alkenes or alkynes, to give l,2-bis(hetaryl) alkanes (or the respective alkenes) [82]. With cyclooctene a 1 1 adduct is obtained instead. Another valuable application is the smooth reaction with ketones (Scheme 26). The thian-threnium salts (40) now obtained are readily deprotonated to the corresponding ylides (41) [83]. The latter compounds are directly obtained when yff-dicarbonyls are used. 

Mann and Cottrell have established that electrochemical oxidation of aliphatic sulfides proceeds through a relatively stable radical-cation 15 localized on the sulfur atom. However, the more stable configuration corresponds to a molecular complex 16 derived from two sulfide molecules according to ESR data. " ° The weak bond that is formed between the interacting sulfur atoms is a two-center three-electron (2c, 3e) bond." Further oxidation results in formation of an ordinary single bond between the two positively charged onium ions. Similar observations were made recently for electrochemical behavior of thianthrene where the ratio of radical-cation to dimer could be easily controlled by concentration and temperature and studied quantitatively using in situ UV/Vis-NIR- and ESR-spectroelectrochemical measurements (Scheme 6). ... 

The formation of the thianthrene cation-radical complex with anisole, that is, (HetH) + ArH (HetH---ArH)+ ... 

Radical 80 has been prepared as its perchlorate salt by anodic oxidation in ethyl acetate in the presence of hthium perchlorate. The reactivity toward nucleophiles of material so prepared was investigated nitrite and nitrate ions give 2-nitrodibenzo[l,4]dioxin although the mechanisms of the reactions are not clear. Pyridine gives 7V-(2-dibenzo[l,4]dioxinyl)pyridinium ion (84). Other nucleophiles acted as electron donors and largely reduced 80 back to the parent heterocycle they included amines, cyanide ion and water. In an earlier study, the reaction of 80 with water had been examined and the ultimate formation of catechol via dibenzo[l,4]dioxin-2,3-dione was inferred. The cation-radical (80) has been found to accelerate the anisylation of thianthrene cation-radical (Section lII,C,4,b) it has been found to participate in an electrochemiluminescence system with benzo-phenone involving phosphorescence of the latter in a fluid system, and it has been used in a study of relative diffusion coefficients of aromatic cations which shows that it is justified to equate voltammetric potentials for these species with formal thermodynamic redox potentials. The dibenzo[l,4]dioxin semiquinone 85 has been found to result from the alkaline autoxidation of catechol the same species may well be in-... 

Use of iodine-silver perchlorate may accomplish cation-radical formation before the oxidizing pair can themselves react. In modern usage, silver ion (as the perchlorate usually) is added to a solution of the substrate and iodine, and the complexity of the iodine-silver perchlorate system is avoided, provided that the substrate undergoes reasonably fast oxidation. Such is the case with perylene (Sato et al., 1969 Ristagno and Shine, 1971) and pheno-thiazine, but not the case with diphenylanthracene and thianthrene (Shine et al., 1972). 

Cation radicals may be oxidized further, and some of them become stable dications. This process is often characterized as the second wave in two successive anodic one-electron oxidations. The thianthrene dication is now well known (Shine and Piette, 1962 Hammerich and Parker, 1973), but has not been isolated. On the other hand, the dipcrchloratc of 2,3,7,8-tetramethoxythianthrene dication is a blue solid (Glass el al., 1973). Dications of some aromatics which are well-known for forming stable cation radicals (e.g. perylene) have been made in FS03H-SbF5 solution (Brouwer and van Doom, 1972). Irradiation of hexachlorobenzcne cation radical in SbFs-Cl2 causes the formation of the dication (in this case, a triplet state) (Wasserman et al., 1974). Oxidation of metallo-porphyrin cation radicals to stable (but reactive) dications is quite common (Dolphin et al., 1973). 

Synthesis and Properties.—The two most general methods for preparing sulphonium ylides continue to be a-deprotonation of a sulphonium salt and the reaction of a sulphide (or disulphide with a carbene. A new development involves the reaction of the thianthrene or phenoxathiin cation radical with a dicarbonyl compound, e.g. ethyl benzoylacetate, to give (4). An infrequently used but useful route to sulphonium ylides involves reaction of sulphides e.g. dithia[3,3]cyclophanes with benzyne. A detailed description of a preparation of the sulphonium salt precursor to Trost s diphenylsulphonium cyclopropylide has appeared. The selectivity of ylide formation in the reaction of cyclic and acyclic sulphides with carbenes has been examined and compared with the much... 

---